Molecular Insights into Scale Inhibition: A Molecular Dynamics Study of Phosphonate-Based Inhibitors on a Calcite Surface

The formation and deposition of mineral scales impose significant challenges to the safety and integrity of field operations across various sectors such as water treatment, desalination, and oil and gas. To mitigate the mineral deposition issues, scale inhibitors (SIs) with phosphonate and carboxylate-based structures are widely employed. Understanding of how these inhibitors interact with minerals and ions is essential for creating more effective, environmentally friendly solutions that meet operational demands. In this study, we employed molecular dynamics (MD) simulations to explore how widely used phosphonate-based SIs adsorb onto calcite surfaces, examining their behavior under different temperatures and salinity levels, both in bulk solutions and within calcite nanopores. Results reveal that the SIs exhibit high compatibility with salty environments, showing low aggregation tendencies. Interestingly, SIs do not form large aggregates in high-salinity brine (213,000 ppm of total dissolved salts), indicating their compatibility with such conditions. MD simulations reveal that phosphonate-based SIs predominantly adsorbed as hydrated outer-sphere surface complexes (OSSC) within the Stern double layer. While the carboxylate-based SIs formed both inner-sphere surface complexes (ISSC) and OSSC with higher adsorption than phosphonate-based SIs. The observed superior performance of phosphonate-based SIs over carboxylate-based SIs can be attributed to two primary factors: reduced loss into reservoir formations due to weaker adsorption and superior thermal stability. Phosphonate-based SIs were found to be significantly more thermally stable than their carboxylate counterparts, which is crucial for high-temperature applications. Diffusion coefficient calculations further indicated that brine salinity has minimal impact on the mobility of SIs, highlighting their robust performance under varying salinity conditions. This study provides detailed atomistic insights into the adsorption mechanisms of SIs on scale-forming mineral surfaces. These findings offer valuable guidance for the development of more efficient and environmentally sustainable scale inhibitors.